The aim of this thesis is to get acquainted with the concept of mass. Classical physics defines mass as the unchangeable property and measure of the body's inertia. The General law of gravity defines a new feature of body attraction and the measure of intensity of attraction which is called the gravitational mass, but the General relativity theory shows the equality of inertial and gravitational mass. Even in the Special relativity theory mass does not depend on the velocity and direction of the movement of the body. Furthermore, by expressing kinetic energy in the Special relativity theory, we get a new body property, known as the rest energy. The rest energy equals the total energy of the system when the body is in the rest. Since it is invariant to Lorentz's transformation, we conclude that the energy of the body's rest is equal to its mass. However, only the Standard Model answers the question of what causes the body mass.